The present invention relates to a process and an apparatus for treating an exhaust gas, more particularly to a process and an apparatus for detoxicating an exhaust gas let out from a chemical vapor deposition (CVD) system for forming silicon epitaxial films, polycrystalline films or amorphous films using silicon-containing gases (silane halide gases) in a semiconductor manufacturing process. The silicon-containing gases refer to halogenosilane gases such as trichlorosilane (TCS: SiHCl3) and dichlorosilane (DCS: SiH2Cl2), and silicon halide gases such as silicon tetrachloride.
The epitaxial (single crystal growth) process, which is a silicon CVD process, is employed for preparation of substrates for field-effect MOS (metal-oxide-semiconductor) transistors or for formation of emitter layers in bipolar transistors. The epitaxial process is generally carried out by using as a raw gas a silicon-containing gas such as TCS and DCS which is diluted with hydrogen before introduction into a process chamber and by heat-decomposing the raw gas by heating the substrate placed in the process chamber to about 1100xc2x0 C. to effect deposition of silicon on the substrate. The above process is generally carried out under a pressure conditions of atmospheric to 100 Pa (Pascal).
Meanwhile, the polycrystal growth process is employed for forming gate electrodes of field effect MOS (metal oxide semiconductor) transistor and ground layers for capacitors. In the polycrystal growth process, a silicon-containing gas such as TCS and DCS is diluted with hydrogen, and the thus diluted gas is introduced into a process chamber in which a substrate heated to about 800xc2x0 C. is loaded to effect heat decomposition of the raw gas to achieve deposition of silicon on the substrate. This processing is usually carried out under a vacuum condition of about 100 Pa.
Further, in such processes, for the purpose of control of moisture to be adsorbed by wafers as they are loaded in and out of the process chamber, a moisture monitor (an optical analyzer for optical measurement; e.g., Fourier Transform infrared (FTIR) spectrophotometer) is occasionally attached to an exhaust piping system.
In such crystal growth process as described above, the amount of the raw gas which is introduced into the process chamber for the purpose of silicon deposition and which contributes actually to the deposition of silicon on the substrate is about 5%, and the most of the rest of the raw gas is exhausted without contribution together with intermediate products (about several % of the total amount) from the chamber. The exhaust gas let out from the chamber is detoxicated by a detoxicating unit which removes the raw gas and intermediate products, and only hydrogen as the carrier gas and nitrogen as the purge gas are released into the atmosphere.
In the epitaxial process, however, there is a problem that intermediate products (by-products) formed during the process adhere or deposit on the inner wall surface of the exhaust piping to be likely to cause clogging of the exhaust piping with the deposit. Further, light transmitting windows of optical analyzers are tarnished with the deposit to make it sometimes difficult to carry out accurate measurement. Such intermediate products include compounds of silicon and chlorine or of silicon and hydrogen, and these compounds form polymers at room temperatures on the inner wall surface of the exhaust piping. The polymers formed are converted to highly reactive (self ignitable or explosive) materials, for example, polysiloxanes, by the moisture contained in the atmosphere. Accordingly, when the exhaust piping is disassembled to be open to the atmosphere in order to remove the polymers deposited on the inner wall surface of the exhaust piping, various preparations and contrivances are required, being causative of dropping the operation efficiency of the CVD system.
Meanwhile, it is practiced to feed an etching gas such as chlorine trifluoride (ClF3) into the exhaust piping in order to prevent deposition of the polymers. It is true, however, that the intermediate products deposited on the inner wall surface of the piping can be removed according to this method, but the method involves a problem in that the exhaust piping itself is corroded by the strong etching property of the etching gas or the etching gas can even cause formation of holes in the piping.
Further, the etching gas such as ClF3 and the raw gas employed in the epitaxial process cannot usually be treated by a single detoxicating unit, so that a plurality of detoxicating units must be used selectively depending on which gas is fed.
On the other hand, there is proposed a method in order to prevent intermediate products from adhering or depositing on the exhaust piping to heat the piping constantly to a temperature of about 150xc2x0 C. According to this method, however, if the temperature of the piping is low at some parts, the intermediate products deposit selectively to such low-temperature parts. The piping between the detoxicating unit and the CVD system usually contains complicated bends from the requirement of reducing the installation area, and it is difficult to heat or heat-insulate the piping uniformly. Actually, maintenance of the piping has been carried out by disassembling the piping to remove the intermediate products deposited at the low-temperature portions.
While a scrubber employing water is frequently used for detoxication of TCS or DCS, solid silicon dioxide (SiO2) is formed by the reaction between water and TCS or DCS, so that the circulation water employed in the scrubber is provided with means for removing SiO2. However, since the thus removed SiO2 contains hydrogen, it cannot be exhausted as such. Thus, it has been practiced to carry out treatment of SiO2 by reacting it with hydrogen fluoride (HF). Since these procedures are carried out as periodical maintenance of the detoxicating unit, not only the operation rate of the CVD system is lowered, but also chemical agents for removing the SiO2 formed, personnel, etc. cost additionally.
It is an objective of the present invention to provide a process and an apparatus for treating an exhaust gas, which can reduce or eliminate periodical maintenance of the exhaust piping and detoxicating units by converting the raw gas employed in the crystal growth process or to highly volatile halides and exhausting the thus obtained halides to the detoxicating unit or a recovery unit without causing adhesion or deposition of the intermediate products in the exhaust piping system.
In order to attain the above objective, in the process for treating an exhaust gas let out from a CVD system for forming a silicon film using a silicon-containing gas according to the present invention, an unreacted raw gas and an intermediate product contained in the exhaust gas are subjected to a decomposition or conversion reaction treatment, and then harmful components contained in the exhaust gas are detoxicated. The decomposition or conversion reaction treatment is carried out by bringing the exhaust gas into contact with a transition metal or a transition metal silicide heated to 400xc2x0 C. or higher. Further, the decomposition or conversion reaction treatment is carried out after addition of hydrogen gas to the exhaust gas.
The apparatus for treating an exhaust gas let out from a CVD system for forming a silicon film using a silicon-containing gas according to the present invention is provided with decomposition reaction means for carrying out decomposition or conversion reaction of an unreacted raw gas and an intermediate product contained in the exhaust gas; detoxicating means for detoxicating harmful components contained in the exhaust gas let out from the decomposition reaction means; and means for heating or maintaining an exhaust gas passage from the CVD system to the decomposition reaction means to or at a predetermined temperature. The exhaust gas passage is provided with hydrogen gas adding means for adding hydrogen gas to the exhaust gas. The decomposition reaction means is provided with a reactor packed with a transition metal or a transition metal silicide and means for heating the transition metal or transition metal silicide to a predetermined temperature.
According to the present invention, since the unreacted raw gas and the like can be decomposed or converted into hydrogen chloride (HCl) which can be treated easily, no deposit is formed on the inner wall surface of the piping, and further the periodical maintenance of removing deposit becomes unnecessary, improving operation rate of the CVD system. Further, since no SiO2 is formed during the detoxication treatment, the load to be applied to the unit in the detoxicating treatment is reduced and the mechanism for removing SiO2 having been installed conventionally becomes unnecessary, resulting in curtailment of the cost of the detoxication treating unit. Further, cleaning of the exhaust piping using ClF3 becomes unnecessary, and thus the cost required for the cleaning can be reduced, and also the exhaust piping can be simplified. In addition, no damping of light occurs when monitored using an optical measuring unit installed in the exhaust piping, so that accurate measurement can be performed and that maintenance of the optical windows in measuring instruments can be eliminated.